The invention relates to a loading arrangement comprising a riser extending from a subsea structure to a coupling element that is attached to the riser for coupling the riser to a vessel, the coupling element comprising a buoy body that is connected to the sea bed via anchor lines that are provided with buoyancy means at or near their ends that are located near the buoy body.
In the Heidrun fields, oil is transferred from the subsea well to shuttle tankers via a Direct Shuttle Loading (DSL) system. In this way intermediate storage facilities need not be used and continuous oil production and transfer directly to the shuttle tanker is possible. The shuttle tankers comprise a submerged tapered loading and mooring construction having a keel cavity in which a coupling buoy is received. The tapered coupling buoy is attached to flexible risers connected to the subsea oil well and is attached to the sea bed via anchor lines. The anchor lines are near their upper ends provided with buoyancy such that the coupling buoy is maintained at a predetermined position below water level upon detaching from the shuttle tanker. Such a system is further described in WO 96/36529.
During high seas, the shuttle tanker will be disconnected from the coupling buoy, for instance at wave heights of 10 m or higher. When the wave height decreases, the shuttle tanker needs to be reattached to buoy at significant wave heights of 4-5,5 m or at higher sea states, which is a very difficult and precise operation. The horizontal and vertical position of the detached buoy, which is suspended between the buoyant upper ends of the anchor lines, is very stable and can not follow the relative movements of the vessel during the hook-up of the tapered buoy. It is therefore an object of the present invention to provide a tapered buoy loading arrangement which can be easily coupled to a shuttle tanker after detachment.
Thereto the loading arrangement according to the present invention is characterised in that the buoy body is connected to a retention member via a flexible connection part, the retention member being attached to the anchor lines, wherein the connection part has a relatively high tensile strength to anchor the vessel to the seabed and to prevent drift of the vessel when tension is exerted on the connection part and the anchor lines. By the substantially flexible connection part, the tapered buoy is decoupled from the relatively large horizontal and vertical stiffness of the anchoring means. The buoyancy means may be formed by the retention member, which may have positive buoyancy or by separate buoyancy members attached to the end of each anchor line, or by a combination thereof. It is not necessary for the buoy body to have a lot of buoyancy. Because of the substantial flexible connection of the tapered buoy to the retention member, the buoy is able to follow the vertical and horizontal movements of the vessel, which makes it easy to pull the tapered buoy in towards the shuttle tanker and to align the buoy with the keel cavity during the hook-up procedure. By providing a substantially flexible connection part, the dynamic vessel is in a flexible way connected to the relatively stiff and stable mooring and loading system formed by the retention member and the anchor lines. With the term xe2x80x9cflexiblexe2x80x9d it is meant a connection which can be displaced in a lateral direction with respect to the vertical such as a chain or cable connection, a pivoting frame or a tubular member which comprises pivoting segments, and the like.
It should be noted that an offshore tanker loading system in which a flexible attachment between a coupling member which is located at the water surface for coupling to a shuttle tanker, and a submerged retention member in the form of a buoy is known from U.S. Pat. No. 5,275,510. In the known loading system however the retention member is connected to the seabed via a riser system. From the retention member a single riser extends vertically upwards to the coupling member for providing a fluid connection with a shuttle tanker. This system can only be used in combination with a dynamical positioning system in which the tanker position is maintained constant by control of the thrusters. No anchoring forces can be transmitted through the vertical riser part towards the seabed such that an anchoring function is not present in this case.
In one embodiment of the present invention, the retention member comprises a chain table connected to the seabed via at least two anchor lines. The chain table may comprise buoyancy to keep it at its desired depth. The anchor lines can near their upper ends be provided with buoyancy members and can extend in a circular pattern around the chain table such that it is maintained at a predetermined depth below sea level, for instance 50 meters at the total water depth of for instance 1400 meters. The chain table may comprise a rotatable swivel having a stationary part connected to the riser and a rotating part connected to a flexible riser section which extends from the rotating part to the tapered buoy. The flexible riser section is attached to the tapered buoy via a second swivel for allowing displacement of the flexible riser section in a plane through the connection part. In this way relative rotations of the vessel with respect to the chain table can be accommodated without exerting too large tensions on the flexible riser section between the chain table and the tapered buoy.
In another embodiment the retention member may comprise a pivot arm that is pivotably connected to a vessel, such as a floating production storage and offloading vessel (FPSO) wherein the connection part is attached at or near the free end of the pivot arm. The pivot arm may comprise a cryogenic transfer boom having two interconnected pivoting pipes. In this system the tapered buoy is permanently connected to the pivot arm and can be easily picked up in the keel cavity of the shuttle tanker for offloading without the buoy being moored to the seabed. This embodiment is particularly useful in harsh environments and during higher sea states of wave heights between 6-8 m, and improves the shuttle tanker connect/disconnect sea state and thus the overall availability of the shuttle tanker. The distance between the first and second vessels could be as large as 500 meters. A ballast weight may be attached to the pivot arm, which in another embodiment may for instance be a delta frame, to stabilize the frame when the tapered buoy is disconnected from the shuttle tanker. A further advantage of attaching the tapered buoy to the pivot arm is that upon connection, the pivot arm keeps the shuttle tanker at a relatively fixed distance from the first vessel (FPSO).
Preferably a weight is attached to a support arm that is located transversely to the pivot arm such that the weight is located below the pivoting connection of the pivot arm when the pivot arm is in its submerged equilibrium position. In this way a stable submerged position is achieved wherein the moment on the pivot hinges is relatively low.
The connection part may comprise a chain which can be provided with a chain swivel for allowing rotation of an upper and lower chain part upon weathervaning of the vessel. It is also possible to use a substantially rigid frame member as a connection part, the frame member being connected to the buoy body via a pivot connection such that the buoy body may be tilted with respect to the frame member upon drift of the shuttle tanker.
In another embodiment the tapered buoy is connected to a first vessel via a flow line which is taken up by a winch on the vessel. The buoy could be moved into the direction of the keel cavity of the shuttle tanker via a hook up line, a remote operated vehicle (ROV) or with thrusters connected to the tapered buoy.